Silver halide imaging elements contain at least one radiation-sensitive silver halide emulsion layer. The emulsion layer contains, as a minimum, silver halide grains in a dispersing medium, typically an organic vehicle, such as gelatin.
Black-and-white silver halide imaging elements, following imagewise exposure, are developed to produce a silver image. Silver halide grains that are not converted to silver in the development process are subsequently removed by fixing.
Color (most typically multicolor) silver halide imaging elements, following imagewise exposure, are developed to produce one or more dye images. In the most common imaging route reduction of silver halide to silver (development) oxidizes a color developing agent which in turn reacts with a dye-forming coupler to produce a dye image. The silver that is produced is an unwanted by-product that is reconverted to silver halide by bleaching. All silver halide is removed from the element by fixing.
Environmental concerns have led to a thorough investigation of the processing of silver halide imaging elements. As most commonly practiced element processing includes development in an aqueous developer solution (or activator solution, when the developing agent is incorporated in the element), immersion in a stop bath which adjusts pH to arrest development, fixing to remove silver halide remaining following development, and rinsing. In color photography developed silver is additionally reconverted to silver halide, which is accomplished using a separate bleaching solution or integrated with fixing by using a bleach-fix (i.e., blix) solution.
At one extreme has been the integration of all processing components into a silver halide imaging element and employing heat to activate processing. Although this eliminates all of the aqueous solutions associated with wet processing, the resulting elements are markedly inferior in their imaging capabilities. This has limited their use to specialized applications where the simplicity of dry processing outweighs overall imaging performance.
Much more effort has gone into examining each of the aqueous processing solutions commonly used and modifying their components to reduce environmental objections. Substantial progress has been realized in providing more environmentally favorable developing solutions, but fixing solutions, despite improvements have remained the primary focus of environmental objections.
The need for fixing a silver halide imaging element following development has been traditionally identified as the need to prevent the silver halide grains remaining after development from printing out (that is, from being reduced to silver). This is seen as objectionably elevated minimum densities.
There is, however, a second reason for fixing out residual silver halide. In an imaging emulsion the silver halide grains have a refractive index much higher than the organic vehicle in which they are dispersed. Silver halide has a refractive index ranging from 2.0 to 2.2, depending upon the specific halide. On the other hand, gelatin, the most commonly employed organic vehicle, has a refractive index of only 1.54. Although individual organic vehicles differ somewhat in their refractive indices, all have refractive indices much nearer to gelatin than to silver halide. Virtually all organic materials have refractive indices less than .+-.10% of the refractive index of gelatin.
Fuji U.K. Specification 1,342,687 (hereinafter also referred to as Fuji '687) suggested that light scatter by image-forming silver halide grains, typically in the 0.3 to 3.0 .mu.m size range, can be reduced by blending silver halide grains having sizes (i.e., equivalent circular diameters or ECD's) of less than 0.2 .mu.m.
Although reducing scatter during light transmission through a silver halide imaging element after processing increases image sharpness, it must also be kept in mind that light scattering during imagewise exposure of a silver halide imaging element has been sought, since it is known to increase imaging speed. Marriage U.K. Specification 504,283, Yutzy et al U.K. Specification 760,775, and Locker U.S. Pat. No. 3,989,527 each add solid particles to increase light scatter, thereby realizing increased imaging speed. When particles are employed for speed enhancement, relatively small concentrations of the particles are effective. For example, Marriage teaches concentrations ranging from 5 to 40 percent for particles having a refractive index of 2.1 or higher. To be effective in scattering light the sizes of the particles must be within .+-.0.20 .mu.m of the wavelength of visible light 400 to 600 nm (0.4 to 0.6 .mu.m). For example, Locker teaches particle sizes ranging from 0.2 to 0.6 .mu.m for scattering visible light.